This invention relates to an organic electroluminescent (EL) device using an organic compound and more particularly, to an organic EL device having a barrier layer between a substrate and an organic EL structure.
Recently, active research works have been made on organic EL devices. As a basic configuration, the organic EL device includes a hole injecting electrode of tin-doped indium oxide (ITO) etc., a thin film formed thereon by depositing a hole transporting material such as triphenyldiamine (TPD), a light emitting layer deposited thereon of a fluorescent material such as an aluminum quinolinol complex (Alq3), and a metal electrode or electron injecting electrode formed thereon from a metal having a low work function such as magnesium. Such organic EL devices are attractive in that they can achieve a very high luminance ranging from several 100 to several 10,000 cd/m2 with a drive voltage of approximately 10 volts.
The organic EL devices sometimes suffer from a decline of luminance with the lapse of driving time, abnormal light emission due to current leakage, and a phenomenon of generating and propagating non-luminous regions known as dark spots. As dark spots propagate and the abnormal light emission worsens, the devices can deteriorate to a practically unacceptable level. Also, the organic EL devices are quite vulnerable to moisture. Penetration of moisture can cause separation between the light emitting layer and the electrode layer or alter the properties of the constituent materials, also creating dark spots and failing to maintain light emission of the desired quality. It is then an important task to prevent the occurrence of defects and the deterioration of device characteristics.
One factor that causes deterioration of the organic EL device is impurities in substrate glass such as sodium and potassium which migrate and diffuse from the substrate to the hole injecting electrode. A common solution to this problem is to use alkali-free glass, which is expensive. It is commercially advantageous if inexpensive alkali glass can be used.
An object of the present invention is to provide an organic EL device which has advantages including minimized occurrence of abnormal light emission and dark spots, improved storage life and durability and which can be manufactured at a low cost.
According to the invention, there is provided an organic electroluminescent (EL) device comprising a substrate of alkali glass, an organic EL structure on the substrate, and a barrier layer between the substrate and the organic EL structure, the barrier layer containing silicon oxide.
In preferred embodiments, the barrier layer has a refractive index of 1.40 to 1.55 at a wavelength of 632 nm; the barrier layer consists essentially of silicon oxide which is represented by SiOx wherein x is from 1.8 to 2.2; the barrier layer is formed by sputtering, especially to a thickness of 5 to 50 nm and a mean surface roughness (Ra) of 2 to 50 nm.
In further preferred embodiments, the substrate contains 10 to 30% by weight, calculated as Na2O and K2O, of sodium or potassium or a mixture of sodium and potassium; the substrate has a mean surface roughness (Ra) of 2 to 50 nm; the substrate has been mirror polished.